The invention relates to a method for determining the amount of air entering a cylinder of an internal combustion engine or series of engines as a function of the air pressure in at least one engine inlet manifold.
One objective is to determine the air mass flow rate of air let into the cylinder to which the aforesaid inlet manifold is connected, in order in particular to define the correct air/fuel mixture, at the same time minimizing emissions of polluting gases.
In this context, one problem that the invention addresses is concerned with obtaining a more appropriate air flow rate to the cylinder(s), using for that a simple, effective but functional method.
To do this, it is already known practice to proceed as follows:
a campaign of bench tests is carried out on the engine or on an engine of the series in order to establish a law governing the evolution of the air mass flow rate MAF entering a cylinder of the engine connected to said inlet manifold, as a function of the admission pressure MAP in this inlet manifold, this being done for various engine speeds, then,
on an operating series engine:
the air pressure in said engine inlet manifold is measured,
a corresponding air pressure MAP value is supplied to a computer which is also supplied with the engine speed corresponding to it,
and, from this, said air mass flow rate is deduced as a function of the established evolution law.
However, it has been found that, at least for some types of engine, there is a discrepancy between the air flow rate value supplied by the aforesaid computer as a function of said evolution law and the actual value of this flow rate, at least for certain values of the admission pressure (pressure MAP) in the inlet manifold.
It will now be noted that this discrepancy occurs at least on xe2x80x9csmall enginesxe2x80x9d, namely gasoline engines, in which the volumes of the inlet manifold(s) are small, and in particular smaller than the cylinder capacity of the engine. It has even been found that a sector to which the solution of the invention more particularly applies is that in which the following relationship is satisfied:                     V        c            xc3x97              N        c              C     less than   3.5
with Vc: volume of the inlet manifold or of the manifold xe2x80x9cplenumxe2x80x9d where the pressure MAP measurements are taken,
Nc: number of engine cylinders
C: cylinder capacity of the engine.
It will be noted for all practical purposes that the term xe2x80x9cplenumxe2x80x9d of an inlet manifold is applied to a chamber interposed on this manifold between the member that regulates the admission air flow rate (typically the butterfly valve) and the inlet valve of the cylinder to which said manifold is connected.
Particularly with this type of engine (although other engines may be affected), a significant feature of the invention therefore anticipates, in order to provide a solution to the aforesaid discrepancies, that, at least for certain values of the pressure MAP, the computer will be supplied, by way of evolution law, with a corrected model. However, given that test campaigns have demonstrated that at least in many cases there is a pressure MAP threshold beyond which the air flow rate MAF no longer evolves as a substantially linear function of the pressure MAP, for various engine rotational speeds N, this being particularly true of engines of small cylinder capacity, the invention proposes, in order for the onboard computer to be able to supply the conditions of functional stoechiometric air/fuel mixture, that:
for a series of engine speeds N in succession, and up to a certain pressure MAP value, said evolution law is established by modeling it using corresponding series of refined straight lines
y=axxe2x88x92b,withxe2x80x83xe2x80x83(a)
y=MAF
x=MAP
a=a1xc3x97N (a1 is a constant)
b=a2xc3x97N (a2 is a constant)
and, from an pressure MAP threshold onward, a corrected model is applied to take account of a difference between the actual value of the flow rate MAF and its value according to the model (a), beyond the threshold, to do that replacing b with bxe2x80x2, with:
bxe2x80x2=f(N, MAP2, MAP3)+a3xc3x97N
so that the evolution in the flow rate MAF becomes a function not only of the engine speed N but also of the evolution to the second and third power of the pressure MAP.
In addition, for a first category of engines, it is even advisable, still in order to take account of the aforesaid discrepancies, that the procedure be as follows:
said law governing the evolution of the air flow rate as a function of the pressure in the inlet manifold is established by considering that:
MAF=S1xc3x97MAPxe2x88x92Ofs(N)
with MAF: air flow rate of the air entering the cylinder,
S1: constant,
N: engine speed,
MAP: air pressure in the inlet manifold connected to the cylinder in question,
Ofs (N): value, at a given engine speed N, of the flow rate MAF at zero pressure MAP,
and, at least for said certain pressure MAP values, a correction is introduced into the value of the flow rate MAF using the corrected model so as to take account of a difference there is between the actual value of the flow rate MAF, at a determined value of the pressure MAP and a given engine speed N, and the one supplied by said established evolution law, this correction being determined from measurements made during the bench test.
Because, on certain engines tested, the evolution in the air flow rate MAF as a function of the admission air pressure MAP, at various engine speeds N, depart markedly from a straight line under certain engine operating conditions, it will be noted that in actual fact it is possible to envisage applying said corrected model to all the pressure MAP values.
In order to obtain the most precise value of the air flow rate MAF, it is, for all that, advised in the invention to supply the computer, at least for certain pressure MAP values, by way of a corrected model, that established according to the law:
MAF[(S0xc3x97MAP3)xe2x88x92(S1xc3x97MAP2)+(S3xc3x97MAP2)]xc3x97Nxe2x88x92Ofs(N)with:
S0, S1, S3: constants,
Ofs (N): value of the air flow rate MAF, at zero pressure MAP and at the engine speed N.
One problem with which the invention is also faced is that of the best way of taking account, if necessary, of the pressure MAP threshold beyond which the discrepancy in the evolution law MAF=f(MAP) is considered to degrade the results.
The solution proposed by the invention is to take account of an averaged filtered pressure MAP. Indeed it is found that the mean of the air pressure MAP is a continuous function of the position of the member that regulates the air flow rate in the inlet manifold (typically the position of the butterfly valve) and of the engine rotational speed.
In fact, it is even advised to define the admission air pressure value (threshold) beyond which the corrected model applies, taking the following into consideration:
said filtered averaged pressure values,
and instantaneous admission air pressure values measured more or less upon opening of the inlet valve inserted between the cylinder in question and the inlet manifold in question.
In series engines in operation, the filtered averaged measurements of pressure MAP will be supplied to the computer for real-time air/fuel metering. It will be noted that, particularly in the case of the aforesaid xe2x80x9csmall enginesxe2x80x9d, the sampling will be performed asynchronously with regards to the rotational cycle of the engine.
Apart from the foregoing, the invention also relates to an electronic computer installed in an internal combustion engine to determine a setting for correct operation of this engine, one feature of the computer being, according to the invention, that in this computer is modeled the law governing the evolution of the air flow rate MAF as a function of the admission pressure MAP, as follows:
for a series of engine speeds N in succession, and up to a certain pressure MAP value, there is established in the computer the law governing the evolution of the air flow rate MAF entering a cylinder (1) of the engine connected to an inlet manifold (5, 7) through which at least air passes, as a function of the air pressure MAP taken from said inlet manifold, by modeling it with a corresponding series of refined straight lines
y=axxe2x88x92b,with:xe2x80x83xe2x80x83(a)
y=MAF
x=MAP
a=a1xc3x97N (a1 is a constant)
b=a2xc3x97N (a2 is a constant)
and, from a pressure MAP threshold onward, a corrected model is applied to take account of a difference between the actual value of the flow rate MAF and its value according to the model (a), beyond the threshold, to do that replacing b with bxe2x80x2, with:
bxe2x80x2=f(N, MAP2, MAP3)+a3xc3x97N
xe2x80x83so that the evolution in the flow rate MAF becomes a function not only of the engine speed N but also of the evolution to the second and third power of the pressure MAP.